Satellite attitude control is traditionally achieved using sensors and actuators to detect orientation errors and provide the necessary control torques to correct the errors. Earth sensors are used to determine the satellite orientation with respect to the earth. Infrared detectors are commonly used to determine the location of the earth's horizon against the cold background of space. Generally, individual photodiodes are employed as the detector(s) along with rotating or scanning mechanisms to allow a periodic sweep of the detector across the space/earth and earth/space boundaries. The centre of the earth is determined as the midpoint between the two horizon transitions.
Satellites are typically stabilized so that their axes have a known relationship with earth to facilitate such activities as earth observation and communication with ground terminals. Certain types of satellites are stabilized by spinning about one of their axes while other types are three-axis controlled so that the orientation of the satellite is maintained as required. Earth sensors are used for both applications and most geosynchronous satellites use spin stabilization at least during the transfer orbit phase prior to deployment on station. In order to satisfy both stabilization scenarios, these satellites usually carry two completely different sets of equipment including both earth and sun sensors. The need for two completely different types of equipment is a major drawback in satellite design because of the added mass.
U.S. Pat. No. 3,551,681 issued to Astheimer is directed to a non-scanning position indicator having at least two separate radiometric cells or detectors each comprising an objective lens, a triangular field mask located in the focal plane of the objective lens, a field lens and a radiation detector. Each separate telescope or cell gathers infrared radiation from two essentially identical fields of view that are from essentially the same region of the earth-space boundary with one of the radiometric cells inverted with respect to the other. The output voltages of the two separate detectors A and B are then electronically ratioed to provide a signal which is linearly proportional to a horizon position. The use of moving parts, namely mechanical choppers is a further drawback to this device. Another major drawback of present earth horizon sensors is that they are usually designed to operate at a specific orbital altitude and therefore the optics are optimized accordingly.
Therefore, it would be very advantageous to provide a satellite attitude sensor system which can function in both spinning and non-spinning modes, which can operate over a wide range of altitudes while still maintaining the required resolution, which is compact, modular, lightweight and does not rely on moving parts.